This invention pertains to silver halide photographic materials, in particular to low pKa release dyes which are useful as the releasable portion of a coupler, wherein the coupler is a one-equivalent coupler. The dyes, when attached to coupler moieties, are also useful as masking couplers in photographic elements. The dyes are also useful as filter dyes in photographic elements.
For economic reasons, it is desirable in the photographic art to reduce the amount of silver used during processing. One way to reduce the use of silver is to use one-equivalent dyes or couplers, which result in the formation of two dye moieties from development of two molar equivalents of silver. Examples of various one-equivalent couplers are found in U.S. Pat. No. 4,840,884 and European Patent Application 173,302. These one equivalent couplers usually do not remain ionized after the film is processed.
The tendency of a dye to remain ionized is related to its acidity, of which the pKa value is a measure. Although, strictly speaking, pKa values relate to dilute aqueous conditions and thus cannot be determined for many hydrophobic photographic dyes, a closely related acidity scale can be constructed using aqueous micelles of a nonionic surfactant, such as Triton X-100 as the reference condition, instead of water. A procedure for acidity determinations in micelles is given in Example 4, and the term pKa used hereinafter refers to acidities determined in such a micelle medium.
As previously mentioned, one-equivalent couplers generally produce an ionizable dye. The dyes have a high pKa, that is, greater than about 5, such that the dye does not ordinarily remain completely ionized after the film is processed. These dyes have disadvantages because partial protonation of the auxochromic group can occur during or after processing. Such protonation may lead to a shift in hue. This shift in hue occurs particularly in dyes with an oxygen/hydroxyl auxochrome, because these dyes have a different hue depending on whether they are ionized or not. These dyes are known as indicator dyes.
To prevent this undesirable shift in hue caused by the dye not remaining fully ionized in the film, it is often necessary to use mordants or a special cation to keep the auxochromic group of the dye fully ionized at a typical gel (film) coating having a pH of about 5.7.
A mordant is known in the art as a material, such as a polymer, which complexes or absorbs the dye and stabilizes the ionized form. However, the use of mordants is not always acceptable because they often have undesirable affects on other photographic properties, such as the reduction of the rate of bleaching and/or fixing, as well as the retention of materials such as sensitizing or filter dyes, which normally wash out of the film.
Low pKa azo dyes have been described in EP-173,302, U.S. Pat. Nos. 4,248,962 and 4,711,837 and Japanese Kokai 63-202745. However, there has not been described in the art other types of dyes which have a low pKa and thus remain fully ionized in film, and which can be released upon reaction of a one-equivalent coupler with oxidized silver halide.
U.S. Pat. No. 4,923,789 discloses structures A-B, where A is a blocking group and B is a dye, particularly a diffusible dye. However, low pKa dyes such as disclosed hereinafter are not suggested.
Thus, there has been a need to provide a photographic element comprising a low pKa dye which can be released from a coupler. There is also a need to provide a one-equivalent coupler which can release such a dye so as to limit the amount of silver halide required during developing. There has also been a need to provide a releasable dye which is useful in photographic films, wherein the dye remains substantially ionized or completely ionized during the coating of the film and in the produced film, without the need of mordants and/or other ion-stabilizing additives. There also has been a need to provide a low pKa dye which is useful in photographic elements, particularly as a filter dye, without the necessity of being released from a releasing compound such as a coupler.